Biomedical electrodes are a primary component of many medical devices, including cardiac pacemakers and defibrillators, deep brain stimulation devices, cochlear implants, peripheral nerve stimulation devices, spinal cord stimulation devices for pain management, and diagnostic tools. The electrode(s) found on the tip of biomedical leads are placed in contact with the appropriate target tissue, and are used to transmit bio-electrical signals to and from the device and target tissue.
A variety of implantable medical devices on the market today utilize conductive electrode coatings comprised of metal oxides or metal nitrides. Depending on how they are deposited, coatings comprised of metal oxides or metal nitrides can have a variety of topographies and morphologies. When used for medical device electrode coatings, metal oxides or metal nitrides are typically formulated with a microscale roughness and/or porosity such that the surface area is significantly increased over that of the uncoated electrode, which lowers the overall electrical impedance. Despite their rough, high surface area topography, however, metal oxide and metal nitride coatings are still mechanically hard compared to the surrounding soft, biological tissue, which is undesirable in the context of a medical device, and particularly a device intended for long-term implantation.
Furthermore, when used with devices that deliver electrostimulation therapies, common metal oxide electrode coatings become increasingly destabilized as the electrode undergoes cycles of biphasic pulse stimulation, due to the build-up of brittle oxide layers at the surface of the electrode. This degradation of the coating presents numerous problems and undesirable qualities for implanted medical device electrodes; these are the potential for tissue injury due to exposure to the delaminated chunks/layers of metal oxide and exposure to potentially harmful non-uniform or higher than usual charge densities caused by the resulting non-uniform electrode surface.
Conductive polymer coatings have the potential to overcome some of the drawbacks associated with traditional metal oxide or metal nitride coatings. For example, conductive polymer coatings derived from poly(3,4-ethylenedioxythiophene) (PEDOT) have been widely used in the electronics industry. Many of the PEDOT-based coatings used in the prior art, however, have limited utility for biomedical leads/electrodes because the processes for applying the coating are broad and non-specific. Even with extensive masking, a cast, dipped, sprayed, or chemical vapor deposition (CVD)-deposited polymeric film cannot easily be localized to the conductive regions or components of a medical electrode.
In addition, cast, dipped, sprayed, or CVD-deposited coatings of PEDOT-derived coatings on metal substrates often confer limited relative improvement in conductivity when compared to the metal alone, and in some cases, the polymeric film can even be insulating, due to a dispersion of leftover solvent throughout the coating. Furthermore because these coating methods apply the PEDOT-derived coating when it is already in a polymeric form, there is little opportunity for electrostatic bond formation and dipole alignment between the PEDOT polymer and underlying metal substrate during the deposition process. As a result, cast, dipped, and sprayed PEDOT-derived coatings typically exhibit limited adhesion to metal substrates.
It is therefore desirable to develop a conductive electrode coating that exhibits greater mechanical, chemical, and electrical stability than the coatings known in the art, that provides excellent electrical conductivity, and that is biologically acceptable for use in medical device applications.